A revamped branch of Marks & Spencer sits just outside Brixton's modernised tube station, a sign of the galloping gentrification of this once gritty south London neighbourhood. But even now you would be hard-pressed to associate Brixton Academy with any sort of glamour. More accustomed to dance all-nighters and rock'n'roll carnage, tonight the Academy is transformed. This sticky-floored den of tinnitus has become a sophisticated pleasure palace as it hosts the finale of Duffy's glittering year.

Arrayed across the back of the stage, along with some of Duffy's six-piece band, is a string section, who play us in with vigour. A sparkly dais sits in front of the musicians, awaiting 5ft 2in of Welsh wind power. After a few bars of 'Rockferry', Duffy finally sashays on - her heels the stuff of chiropractors' nightmares, her little red dress the stuff of confectioners' nocturnal fantasies. She looks like a predatory bonbon. The scene could be Vegas or Paris or any TV studio in 1964; you half expect Dick Clark - the plastic host of the vintage US TV show American Bandstand - to appear stage right, applauding wildly.

Mindful of injury, Duffy picks her way purposefully across the stage. She keeps her voice teasingly in check for a couple of verses, then finally lets rip on the key change, stripping the paint off the circle. An appreciative shiver runs through the crowd, a mix of ages, genders and sexualities that attests to the commercial reach of this retro diva.

A year of hard touring has done nothing to blunt the force of Duffy's pipes; if anything, they have become more steely. As a performer, she is more polished too, swinging her microphone around like a handbag while the band take a rare solo. There is little movement to her - just an arm, aimed at the sky or the floor, or a pert bottom wiggle.

She leaves the dancing to two limber men in suits, who - sadly - don't lift Duffy up and carry her around as they would in an old-fashioned musical number. In a recent interview Duffy told of her plans for a Forties-themed party, celebrating an era when men 'were men' and women were 'all tits and teeth'. She is reviving the latter half of that tradition with gusto.

This second of two sold-out Brixton gigs ends a musical year which officially began with the release of her debut single 'Rockferry' last December. It has taken in a maiden residency at London's minuscule Pigalle Club last January; relentless touring, including 15 separate trips to New York, a No 1 single and the admiration of Tom Jones, slightly up the pecking order from God in Wales. Once the figures are finalised, 2008 should culminate with Duffy having the biggest-selling album of the year in the UK, unless Take That have a late surge. Not bad for a girl from north Wales who would have replaced Cerys Matthews in Catatonia had the band carried on.

Between songs, Duffy is still finding her feet, caught between registers. 'Is it Tuesday night?' she inquires, shrilly. 'Tuesday night was bingo night in my dad's pub!' A cab driver told her that Brixton used to be the haunt of musicians and actresses, but she's not too sure which ones. Somewhere, though, a switch flicks. She commands 'after you, strings', like a seasoned band leader, and 'Delayed Devotion' swings into life.

Although she only has one album to her name, Duffy's 17-song set boasts a few unfamiliar tunes. Most memorable is 'Rain on Your Parade', a sassy bit of vitriol whose insistent rhythms incorporate the lessons of 'Mercy', her biggest hit. A quick tally reveals that the set list consists of the same 17 tracks as the recently released deluxe edition of Rockferry.

She dedicates possibly her oldest song, 'Enough Love', to manager-cum-mentor Jeanette Lee, who heard it as a demo and decided to take the unformed singer under her wing. Yearning for enough love 'to go round the world', it sounds like a Sixties soul cover version. In fact, it was written by Catatonia's Owen Powell and Richard Parfitt from Welsh pop also-rans the 60 Ft Dolls - just one aspect of the uncommon sequence of power-broking that made Duffy a star.

Powell and Parfitt passed the demo on to Lee, the most unlikely of pop Svengalis. Enmeshed in west London's punk circles, she was an administrative member of Public Image Ltd, John Lydon's post-Pistols band. After raising a family, she joined forces with Geoff Travis at label-cum-management company-cum record shop Rough Trade. She introduced Duffy to former Suede guitarist Bernard Butler, who crafted the rich retro soundbed of Rockferry.

Duffy, then, is an international phenomenon built by former punks and indie kids. It's a shame they didn't make her slightly better. While there is no faulting songs like 'Warwick Avenue' - a treat from Duffy's a capella introduction until the final shimmer - too often her performances lack light and shade. Her voice stuns at full-pelt but rarely catches or swoops. Her customary finale, the giddy, yearning 'Distant Dreamer', leaves you feeling that she should have more songs like this. But Duffy's bonus-disc tunes are full of polite cliches, like 'Fool For You' and 'Please Stay'.

You desperately want Duffy to break a sweat, to really electrify like the vintage acts she admires. Froideur has an important part to play in Sixties-period sexpottery, but abandon - crucial to soul, pop and R&B - is largely absent from Duffy's affective palette. Adele's got it, Beth Ditto's got it, Amy Winehouse has got it, perhaps to a fault. Instead, Duffy's is a kind of M&S version of soul - pleasant, well-made but ultimately overly sanitised.